1. Field of the Invention
The present invention relates to a rechargeable lithium battery, and more particularly to a rechargeable lithium battery which utilizes the improved active material for its positive or negative electrode.
2. Description of Related Art
In recent years, rechargeable lithium batteries have been extensively developed. The performance characteristics of rechargeable batteries, such as charge-discharge voltages, charge-discharge cycle life characteristics and storage capabilities, depend largely on the particular electrode active material used. This has led to the search of various active materials.
The use of titanium oxides for the active material has been investigated. Among them, spinel-phase Li(Li1/3Ti5/3)O4, because of its reduced tendency to be strained during charge and discharge, has been proposed as an active material capable of providing excellent cycle life performance (See for example, T. Ohzuku, Solid State Ionics, 69:201, 1994). Another titanium oxide known as exhibiting charge-discharge activity is anatase-form titanium oxide and its use as an active material has been investigated for years (See, for example F. Bonino, J. Power Sources., 6:261, 1981). It is known that the theoretical capacity is 174 mAh/g for the spinel-phase Li(Li1/3Ti5/3)O4 active material and 335 mAh/g for the anatase-form titanium oxide active material. It is also known that the theoretical capacity of anatase-form titanium oxide is greater than that of LiTiO2, 308 mAh/g.
However, the anatase-form titanium oxide shows a tendency to become inactive after repetitive charge-discharge cycling, leading to the reduction of battery voltage (See, for example, F. Bonino, J. Power Sources., 6:261, 1981).
Japanese Patent Laying-Open No. Hei 6-275263 (1994) discloses that the use of lithium titanate, as prepared by heat treating a combination of titanium oxide and a lithium compound, for negative active material of rechargeable lithium batteries results in the improvement of cycle characteristics. However, lithium titanate exhibits a reduced capacity per gram of active material, compared to the anatase-form titanium oxide, which has been a problem.
There accordingly has been a need for the active material which can yield high charge capacity, comparable to that of anatase-form titanium oxide, undergo little strain during charge and discharge and impart excellent charge-discharge cycle characteristics.
The present invention has been made to satisfy the aforementioned need and its object is to provide a rechargeable lithium battery which exhibits a high capacity and excellent charge-discharge cycle characteristics.
The rechargeable lithium battery of the present invention has a positive electrode, a negative electrode and a non-aqueous electrolyte. Characteristically, the positive or negative electrode contains, as active material, complex oxide represented by the compositional formula MxTi1xe2x88x92xO2 and including an anatase-form crystal structure phase, wherein M is at least one metallic element selected from V, Mn, Fe, Co, Ni, Mo and Ir and x satisfies the relationship 0 less than xxe2x89xa60.11. The complex oxide may further contain lithium.
In accordance with the present invention, the introduction of the metallic element M (at least one of V, Mn, Fe, Co, Ni, Mo and Ir) into the crystal lattice of anatase-form titanium oxide serves to stabilize the crystal structure of the active material. The use of the complex oxide for the positive or negative electrode thus results in the improvement of charge-discharge cycle characteristics.
Any of the afore-listed metallic elements M is known to form a stable compound with oxygen and also to have a decomposition temperature of not below 700xc2x0 C. (See, for example, binary phase diagrams for M-O in xe2x80x9cBinary Alloy Phase Diagramsxe2x80x9d, American Society for Metals, 1986). Since any of these metallic elements M tends to be chemically bound to oxygen by a relatively strong force, such a metallic element M is believed to stabilize the crystal structure of anatase-form titanium oxide when it substitutes for a part of Ti present therein and thus occupies certain sites of the crystal lattice.
In the present invention, the stoichiometry x of the metallic element M in the above-specified composition of the complex oxide is maintained not to exceed 0.11. If the inclusion of the metallic element M is excessive, i.e., if x exceeds 0.11, a separate phase composed principally of M may be produced to result in lowering the improving effect of charge-discharge characteristics.
In the present invention, the complex oxide for use as the positive or negative active material includes a phase of anatase-form crystal structure. The presence of anatase-form crystal structure can be identified by X-ray diffraction (XRD).
An electrolyte solvent for use in the rechargeable lithium battery according to the present invention can be selected from non-aqueous electrolyte solvents generally used for rechargeable lithium batteries. Specifically, it may be a mixed solvent of cyclic carbonate and chain carbonate, for example. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate and butylene carbonate. Examples of chain carbonates include dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate. The electrolyte solvent may alternatively be a combination of the afore stated cyclic carbonate and an ether solvent, for example. Examples of ether solvents include 1,2-dimethoxyethane, 1,2-diethoxyethane and the like. A useful electrolyte solute may be LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2) (C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3 and any combination thereof, for example. Other applicable electrolytes include gelled polymer electrolytes wherein a liquid electrolyte is impregnated in polymers such as polyethylene oxide and polyacrylonitrile, and inorganic solid electrolytes such as LiI and Li3N.
In the present invention, any non-aqueous electrolyte can be used, so long as it contains a lithium compound as a solute for realizing an ionic conductivity and a solvent used to solubilize and hold the solute is hardly decomposed at voltages during battery charge, discharge and storage.
In the case where the aforementioned titanium complex oxide is used for the positive active material, a suitable negative active material may be chosen from carbon materials capable of electrochemical storage and release of Li, such as graphite (either natural or synthetic), coke, and calcined organics; Li alloys such as Lixe2x80x94Al, Lixe2x80x94Mg, Lixe2x80x94In, Lixe2x80x94Alxe2x80x94Mn alloys; and metallic Li. In such instances, a charge voltage of about 3 V and discharge voltage of about 2 V will be given. The contemplated effect of improving cycle life performances becomes more significant when the carbon materials, among those active materials, are used for the negative active material. This is because the carbon materials are contrary in property to the Li alloys and metallic Li which, during charge and discharge, are likely to be accompanied by the growth of treelike dendrites that could cause internal short circuits.
In the case where the aforementioned titanium complex oxide is used for the negative active material, lithium-containing transition metal oxide, such as LiCoO2, LiNiO2, LiMn2O4, LiMnO2, lithium-containing MnO2, LiCo0.5Ni0.5O2, LiNi0.7Co0.2Mn0.1O2 or the like, may be used as the positive active material. In this instance, a charge voltage of about 2.8-3 V and a discharge voltage of about 1.8-2.0 V will be given. The use of titanium complex oxide including the anatase crystal structure phase for the negative active material thus results in a marked improvement of charge-discharge cycle life characteristics. This is considered due to the increased potential of the negative electrode, relative to that of lithium, lithium alloy or Lixe2x80x94GIC (Li-intercalated graphite), which suppressed the reductive decomposition of the electrolyte solution.